Currently, lithium ion secondary batteries have been widely used mainly in electronic devices such as portable devices, since the lithium ion secondary batteries have higher electric voltage, larger charge-discharge capacity, and more resistive to adverse effects due to memory effect or the like, compared to nickel-cadmium batteries.
On the other hand, lithium ion secondary batteries have excellent energy densities and output densities, and are used as batteries of hybrid vehicles and electric vehicles as well as batteries of various portable electric devices such as laptops and mobile phones. In addition, lithium ion secondary batteries are expected to be applied in electric cars and electric power storage unit in future. However, there is a room for improvement of safety and high temperature resistance for lithium ion secondary batteries.
Recently, attention has been paid to non-combustible metallic oxide Li4Ti5O12 as a novel negative electrode active material that can replace carbon negative electrode so as to improve safety and high temperature resistance of lithium ion secondary batteries. Metallic lithium does not precipitate during insertion and extraction reaction of lithium ions in the Li4Ti5O12 since the insertion voltage of lithium ion shows high and flat value around 1.55V vs Li/Li+. Therefore, solid electrolyte interface (SEI) having inferior thermal stability does not tend to be formed on the surface of the electrode. Further, since the change of volume is minimal during the insertion and extraction reaction of lithium ions, Li4Ti5O12 exhibits quite satisfactory cycle properties. Therefore, the use of Li4Ti5O12 in the negative electrode allows designing a battery having higher safety than a battery including a negative electrode constituted of a carbon material.
However, it is difficult to synthesize a single phase product composed only of Li4Ti5O12. During its synthesis, Li4Ti5O12 is obtained as a mixture with Li2TiO3 and rutile type TiO2 (hereafter referred to as “r-TiO2”) that participates in deterioration of battery property. It is generally known that synthetic conditions for Li4Ti5O12 having a specific stoichiometric composition is restricted to very narrow ranges, and Li4Ti5O12 is obtained as a mixture mixed with r-TiO2 or Li2TiO3 depending on the proportions of lithium and titanium (Non-Patent Document 1: G. Izquierdo and A. R. West. “Phase Equilibria in the System Li2O—TiO2.” Mat. Res. Bull., vol. 15, pp. 1655-1660. (1980)). Li4Ti5O12 exists in a mixture with these compounds in previously reported articles and commercial products. In addition, since electron conductivity of Li4Ti5O12 is as low as 10−13 Scm−1, the use of Li4Ti5O12 as a negative electrode material has included a problem of reduction of electrical capacity in the time of discharge with large electric current.
In order to solve the above-described problem, techniques have been proposed to improve battery properties, for example, by combining Li4Ti5O12 with carbon (Non-Patent Document 2: L. Cheng, X. L. Li, H. J. Liu, H. M. Xiong, P. W. Zhang, Y. Y. Xia. “Carbon-Coated Li4Ti5O12 as a High Rate Electrode Material for Li-Ion Intercalation.” J. Electrochem. Soc., vol. 154, pp. A692-A697. (2007)) or silver (Non-Patent document 3: S. Huang, Z. Wen, J. Zhang, Z. Gu, X. Xu. “Li4Ti5O12/Ag Composite as Electrode Materials for Litium-Ion Battery.” Solid State Ionics, vol. 177, pp. 851-855. (2006)).
Publication of International Application WO2010/052950 describes a technique including: adding predetermined amounts of dicarboxylic acid having carbon number of 4 or more, lithium salt, and titanium alkoxide under the presence of water; stirring and dissolving the compounds to obtain a solution; spraying and drying the solution by spray-dry method to obtain a precursor material; firing the precursor material for a predetermined time period at a temperature of 700 to 900° C.; and thereby producing a composite of carbon and non-stoichiometric titanium compound shown by chemical formula of Li4+xTi5−xO12, where x is in the range of 0<x<0.3. WO2010/052950 describes that the non-stoichiometric titanium compound produced by the above-described method has excellent crystallinity and does not include impurity materials such as r-TiO2 and Li2TiO3, and that the use of negative electrode active material constituted of a carbon-containing composite including this non-stoichiometric titanium compound can improve charge discharge property of a lithium ion secondary battery.